FIG. 2 shows a structure of a conventional aluminum electrolytic capacitor. The drawing is a partially cutaway view in perspective thereof. In this structure, capacitor element 19 is formed in the following manner. An aluminum foil is etched so that the effective surface area thereof is increased. A dielectric oxide film is formed on the surface of the etched aluminum foil by chemical treatment so that positive electrode foil 11 is formed. Negative electrode foil 12 is prepared by etching the aluminum foil. Positive electrode foil 11 and negative electrode foil 12 are wound with separator 13 sandwiched therebetween. In capacitor element 19, positive electrode lead 15 is connected to positive electrode foil 11, and negative electrode lead 16 is connected to negative electrode foil 12. Capacitor element 19 is impregnated with a driving electrolyte solution (not shown). Capacitor element 19 is inserted into metallic case 18, such as an aluminum case, and case 18 is sealed with sealing material 17, such as a rubber. Thus, a conventional aluminum electrolytic capacitor can be obtained.
The examples of the material of separator 13 include cellulose fibers, such as Manila hemp, craft, hemp, and esparto. These materials are selected according to the performance, such as thickness and density, as separator 13.
The performance required to the aluminum electrolytic capacitor includes large capacitance, small equivalent series resistance (ESR), and high reliability.
With recent promotion of a size reduction and performance increase in digital circuits in audio visual equipment, personal computers, or the like, increasing capacitance and reliability and decreasing ESR of the aluminum electrolytic capacitor have become more important. To increase capacitance and improve ESR characteristics, studies are made on the material and thickness of the separator and the improvement of the electric conductivity of the driving electrolyte solution.
Patent Documents 1 and 2 are known as the information on the conventional techniques related to the above, for example.
However, a low-density separator used in order to improve the ESR characteristics as shown in the conventional aluminum electrolytic capacitor has a low tensile strength. For this reason, ruptures or displacement occurs when the capacitor element is made by winding, and the stress exerted on the separator by the positive electrode lead connected to the positive electrode foil and the negative electrode lead connected to the negative electrode foil weakens the separator. These phenomena cause short circuits and deteriorate withstand voltage.
Further, when the thickness of the separator is increased with the density thereof kept low for improvement of the strength of the separator, the volume of the electrode foils per unit volume of the capacitor element is decreased in addition that the ESR characteristics are deteriorated. As a result, a decrease in the area of the electrode foils makes the capacitance increase difficult.
Patent Document 1: Japanese Patent Unexamined Publication No. H08-273984
Patent Document 2: Japanese Patent Unexamined Publication No. 2000-173862